Heretofore, various means have been employed and proposed for the production of inorganic pigments comprising complex oxides.
One general method for the production comprises mixing oxides containing elements that constitute specific inorganic pigments, or compounds that can be converted into such oxides under heat, in predetermined ratios, then calcining the resulting mixtures into complex oxides, and grinding the complex oxides.
With this method, however, since the solid state reaction during the sintering starts from the contact points of the starting particles, the intended solid state reaction is often uneven and incomplete, resulting in uneven color and tone in the pigments obtained. Therefore, in order to obtain homogeneously and satisfactorily colored pigments according to this method, the heating temperature must be increased and the heating time must be greatly prolonged. However, this not only adds to energy costs but also enlarges the sintered particles because of the lapse of heating time, resulting in further mechanical energy being expended in the step of grinding such large particles. This method is problematic in such points.
Another known method comprises wet-milling the starting materials for a long period of time to produce a satisfactorily homogeneous mixture of ultra-fine particles, followed by calcining this mixture. In this method, heating time can be shortened to some degree. However, the introduction of impurities, caused by the abrasion of the milling tank and the milling media used, into the mixture is inevitable, and the impurities negatively influence the quality of the pigments thus obtained.
In yet another known method, a flux of fluoride, a chloride or the like is used for the purpose of lowering the heating temperature for the calcining of powdery raw materials. However, this is problematic in that the raw materials are sintered very unevenly and the sintered particles grow too much.
In still another known method, a co-precipitation of particles is used. For example, a means of co-precipitation is disclosed in Japanese Unexamined Patent Publication No. 3-8728, in which more homogeneous pigments may be obtained through co-precipitation of particles. However, the disclosed method comprises complicated steps whereby production costs are increased.
One object of the present invention is to provide a simple method for producing homogeneous, satisfactorily colored, inorganic pigments comprising complex oxides.
Another object of the present invention is to provide a simple method for producing homogeneous, satisfactorily colored, inorganic pigments comprising complex oxides, in which the calcining step is effected at a low temperature for a shortened period of time without overly enlarging the sintered particles, and where the grinding step is thereby simplified.
Yet another object of the present invention is to establish a method for producing homogeneous, satisfactorily green-colored, inorganic pigments comprising complex oxides of titanium, cobalt, nickel and zinc, through calcining of raw materials at a low temperature for a shortened period of time.
Still another object of the present invention is to establish a method for producing homogeneous, satisfactorily yellow-colored, inorganic pigments having a rutile structure that comprises a solid solution of titanium oxide with one or more oxides of cobalt, chromium and nickel and one or more oxides of antimony, tungsten and niobium, through calcining of raw materials at a low temperature for a shortened period of time.
Still another object of the present invention is to establish a method for producing homogeneous, satisfactorily colored, highly transparent cobalt blue pigments, through calcining of raw materials at a low temperature for a shortened period of time.